An enzyme is a biological catalyst that accelerates selected chemical reactions under cellular conditions.

Most enzymes are proteins, although some RNA molecules also have catalytic activity.

Substrates and active sites

The molecules transformed by an enzyme are its substrates. They interact with a region of the enzyme called the active site.

Binding depends on many weak interactions acting together, including electrostatic interactions, hydrogen bonding, dispersion forces and interactions with the surrounding solvent.

The active site is therefore not simply a rigid lock whose shape alone determines the reaction. Enzymes are flexible molecules, and binding can change the conformations of both enzyme and substrate.

Lowering the activation barrier

An enzyme provides a reaction pathway with a lower activation barrier. It can do this through mechanisms such as

• bringing reacting groups into favorable positions;
• stabilizing charge distributions that develop along the reaction path;
• donating or accepting protons;
• temporarily forming covalent intermediates;
• creating a local chemical environment different from the surrounding solution.

A useful way to summarize the effect is

same reactants and products
        ↓
lower kinetic barrier
        ↓
faster approach to equilibrium

The enzyme does not make the products thermodynamically more favorable and does not change the equilibrium constant of the overall reaction.

Specificity

Enzymes can distinguish among similar substrates because productive catalysis requires a compatible combination of geometry, interactions and reaction mechanism.

Specificity is not necessarily absolute: some enzymes act on families of related substrates, while others are highly selective.

Saturation

At low substrate concentration, increasing substrate availability can increase the reaction rate because more enzyme molecules encounter substrate.

At sufficiently high concentration, active sites spend most of their time occupied. The rate then approaches a finite catalytic capacity rather than increasing indefinitely.

This saturation behavior motivates quantitative enzyme-kinetics models developed in more advanced biochemistry.

Cofactors and coenzymes

Some enzymes require additional non-protein components. Metal ions or organic coenzymes can carry electrons, chemical groups or other functionality that the amino-acid side chains alone do not provide.

Enzymes make controlled cellular chemistry possible by selecting and accelerating particular routes through a much larger network of chemically possible reactions.